Urea is produced from ammonia and carbon dioxide. Today's urea production involves relatively clean processes, particularly low in the emission of urea dust and ammonia. However, besides the chemical synthesis of urea, the production of urea on a commercial scale requires that the urea be presented in a suitable solid, particulate form. To this end, urea production involves a finishing step in which a urea melt is brought into the desired particulate form, generally involving any one of prilling, granulation, and pelletizing.
Prilling used to be the most common method, in which the urea melt is distributed in a prilling tower and the droplets solidify as they fall down. However, the end-product is often desired to have a larger diameter and higher crushing strength than the one resulting from the prilling technique. These drawbacks led to the development of the fluidized bed granulation technique, where the urea melt is sprayed on granules that grow in size as the process continues. Prior to the injection in the granulator, formaldehyde is added to prevent caking and to give strength to the end-product.
The air that leaves the finishing section contains urea dust and ammonia. The latter is particularly caused by an unwanted side-reaction in the finishing step, viz. the formation of biuret, i.e. a dimerization of urea, with release of ammonia. Another side-reaction that may occur is hydrolysis of urea, again with release of ammonia. Thus, despite the relatively clean nature of the urea synthesis, the commercial production of urea inevitably goes with the formation of ammonia. This ammonia is normally emitted through the off-gas of the finishing section of a urea plant.
With a view to increased demand for urea production, and increased legal and environmental requirements as to reduce the level of emission of ammonia, it is desired that the ammonia specifically emitted in urea finishing, be prevented or removed. This is particularly challenging, since the amounts of off-gas (mainly air) are enormous, and the concentration of ammonia is low. A typical airstream is of the order of 750,000 Nm3/h. A typical concentration of ammonia therein is 100 mg/Nm3.
The relatively low concentration of ammonia means that the off-gas of a finishing section of a urea production plant does not lend itself to ammonia removal by conventional techniques such as wet scrubbing. Rather, the state of the art in the present field is the removal of ammonia by means of acid. Whilst this leads to very efficient removal of ammonia, it presents a serious drawback in that it results in a by-product, viz. the corresponding ammonium salt. This itself needs then to be disposed of, i.e. the emission problem is effectively traded for another problem of chemical waste.
It is therefore desired to provide a method by which the ammonia from the off-gas of a finishing section of a urea production plant can be removed without causing the formation of a new by-product.
Other methods of removing ammonia from gas are known. Background references include the following.
Helminen et al., AIChE Journal August 2000, Vol. 46 No. 8, pages 1541-1555 presents a comparison of adsorbents and isotherm models for ammonia separation by adsorption. Helminen recognizes that ammonia-gas separation by adsorption and recovery for re-use is well-known, but has not been applied extensively. In this respect reference is made to problems related to the selectivity, capacity and regenerability of the adsorbents. It is indicated that most applications of adsorbents are related to the separation of ammonia from the gas streams in the production process of ammonia. Zeolite, alumina, silica gel and active carbon are mentioned as being used for this purpose. Helminen thereupon presents a study into which of these adsorbents would work best. Ammonia was considered to adsorb most strongly on certain zeolites (13X and 4A). The thrust of Helminen's study, however, is to identify the best models for studying adsorption isotherms, and no practical use of the adsorbents is suggested.
In WO 00/40324 a method is disclosed for the separation of ammonia gas and a solid adsorbent composition. It outlines several problems with the removal of ammonia from gas streams, discussing wet scrubbing, including absorption into acid solutions with the formation of salts as mentioned above, as well as gas adsorption over a solid adsorbent bed with regeneration of adsorbent. In the latter case, an indicated problem is that ammonia is adsorbed very strongly onto many conventional adsorbents such as zeolites, alumina, and silica gel, which is said to cause the adsorption isotherms to be unfavorable for desorption, making adsorbent regeneration difficult. This is said to be different for activated carbon, which can be regenerated simply by depressurizing, but the activated carbon comes with a low selectivity and adsorption capacity for ammonia. As a solution, the reference presents the use of a solid copper (I) containing adsorbent.
Bernal et al., Bioresource Technology 43 (1993), 27-33 relates to certain zeolites as adsorbents for ammonia and ammonium. This reference too focuses on the pertinent adsorption isotherms. The desorption of ammonia, and regeneration of the adsorbent, is not addressed.
A further peculiarity of the off-gas of a finishing section of a urea production plant, is that it not only comprises ammonia, but also urea dust. It would be desired if a method could be provided that would not only serve to remove ammonia from the off-gas, but also urea dust.
All in all, it is desired to provide a method of removing ammonia specifically from a finishing section of a urea production plant. It is moreover desired that this goes without the formation of yet another by-product. It is further desired that the method enables the removal of urea dust from the off-gas as well.